1. Field of the Invention
The present invention relates to a preload device of a ball bearing by a coil spring and a motor having the preload device.
2. Description of the Related Art
When a ball bearing is used as a bearing of a rotational shaft of a motor and the like, if an internal clearance in the bearing exists, a play of a ball is large and stiffness of the bearing is weak. As a result, the rotational vibration of the shaft is increased. Therefore, a preload structure has been known which applies preload to the bearing so as to decrease the play of the ball.
For example, JP-UM-A-H4-82425 describes a preload structure where an outer ring is mounted to a rolling groove provided to a shaft via rolling elements, and preload is applied to the outer ring in an axial direction, thereby reducing the play of the rolling elements.
In such preload structure of applying the preload to the ball bearing, elastic force of a coil spring is generally used. However, in a preload structure where only the coil spring is provided around a shaft of a motor and the like, and the ball bearing is directly preloaded by an end surface of the coil spring, it is necessary to flatten the end surface of the coil spring so as to uniformly apply the preload. In addition, when assembling the rotational shaft and the ball bearing, it is difficult to perform the assembling because the ball bearing is pressed and thus returned due to the elastic force of the coil spring returning to its free length.
Accordingly, in the ball bearing device described in JP-UM-A-H4-82425, an inner cylinder, which accommodates therein a coil spring, is covered with an outer cylinder and an outer peripheral surface of the inner cylinder and an inner peripheral surface of the outer cylinder are configured to be slidable in an elastic direction of the coil spring, i.e., in an axial direction. In addition, after the coil spring is forcibly compressed by pressing the inner and outer cylinders from both sides, the outer and inner cylinders are locked by lock parts of the outer and inner cylinders and are assembled between bearings. Then, after the outer and inner cylinders are assembled between the bearings, by releasing the lock state, preload is applied to the bearings. Accordingly, since the end surfaces of the coil spring are not directly contacted to the bearings, it is not necessary to flatten the end surfaces. However, when the bearing device is provided in a cylinder, it is not possible to release the lock state, and therefore, the elastic force of the coil spring is still an obstacle in assembling the shaft and the bearing. Further, since the coil spring is enclosed in the inner and outer cylinders, it is not possible to recognize whether or not the coil spring is provided or a type of the coil spring at a later time, so that it is not possible to appropriately apply the preload.